Mohs Hardness Scale Glass Rating: What 5.5 Means

On the Mohs hardness scale glass sits at about 5.5, just above apatite at 5 and just below orthoclase feldspar at 6, and that one number turns an ordinary pane into one of the most useful tools a crystal collector can own. Anything harder than glass can leave a scratch in it. Anything softer cannot, however hard you press. Below you will find what the scale actually measures, how to run a careful glass scratch test, why glass outranks ordinary steel but not every blade, and what all of this means for storing and carrying the stones you love.

Where Does Glass Sit on the Mohs Hardness Scale?

First, a quick definition. Mohs hardness describes how well a material resists being scratched, ranked from 1 (the softest, talc) to 10 (the hardest, diamond). It is not about weight, density or how easily something shatters. It is simply a question of which material can leave a visible groove in which.

Most reference tables give ordinary window and bottle glass a value of about 5.5, and that Mohs hardness scale glass rating is the one you will see quoted in field guides. Glass is not a mineral in the strict sense. Minerals are naturally occurring solids with an orderly internal structure, while glass is an amorphous solid: its atoms are frozen in place without the repeating pattern of a crystal. It is made largely of silica, the same compound that forms quartz when it has the time and conditions to crystallise slowly in the earth (our guide to how crystals are formed covers that process). Even without a crystal lattice, glass can be placed on the scale by comparing it with the reference minerals, just like anything else.

Glass is not one single recipe, though. Specialty glasses, such as those made for cookware, lab equipment or device screens, can be a little harder or softer than plain window glass, which is why some guides give a wider spread of values. For everyday purposes, treat the Mohs hardness scale glass figure of 5.5 as a reliable working number and allow a little wiggle room for stones that sit right on the border.

The practical rule follows neatly from it:

• Stones rated 6 or higher should scratch glass.
• Stones rated 5 or lower should not.
• Stones around 5.5 to 6, such as opal, are borderline and may barely mark it or not at all.

What Does Mohs Hardness Measure?

The scale was introduced in 1812 by the German mineralogist Friedrich Mohs as part of a method for identifying minerals in the field. The idea of ranking stones by which one scratches which is far older. Theophrastus described it in his work on stones around 300 BC, and Pliny the Elder returned to it in his natural history around AD 77. What Mohs added was a fixed ladder of ten common reference minerals, so anyone could compare an unknown sample against the same benchmarks.

To find a material’s hardness, you look for the hardest reference mineral it can scratch and the softest one that can scratch it. If a sample is scratched by apatite but not by fluorite, it lands between 4 and 5. In technical terms, a scratch means a permanent mark visible to the naked eye. Softer materials can sometimes cause microscopic damage to harder ones, but that does not count for a Mohs number.

One point surprises many beginners: the Mohs scale is ordinal, meaning it puts things in order without promising equal steps between them. Measured with an instrument, corundum (9) is roughly twice as hard as topaz (8), while diamond (10) is about four times harder than corundum. The table below shows the ten reference minerals with their chemical formulas and an approximate absolute hardness value, with the Mohs hardness scale glass value slotted in where it belongs.

Each of these stones has its own story. Our guide to the minerals on the Mohs hardness scale walks through all ten, and there are deeper looks at topaz at 8, sapphire at 9 and diamond at 10.

Which Crystals Scratch Glass and Which Don’t?

Because glass falls so neatly in the middle of the ladder, it splits familiar crystals into two clear groups. This is the real reason the Mohs hardness scale glass benchmark shows up in every field guide: one quick test tells you which half of the scale a stone belongs to.

Read the table from top to bottom, soft to hard: the stones listed above the Mohs hardness scale glass row are softer and will not mark the plate, while the stones listed below it, from quartz to diamond, are harder and will leave a scratch. Opal sits right on the border.

The soft side: below glass in hardness

Selenite, the silky form of gypsum, sits at just 2. In crystal traditions it is linked with clarity and calm and is often kept on shelves or windowsills, but its softness means it marks easily, even with a fingernail. Our guide to using selenite crystals explains why it is best cared for dry. Calcite at 3 and fluorite at 4 are also too soft to scratch glass, which makes the Mohs hardness scale glass test a handy way to tell them apart from harder lookalikes.

The hard side: above glass in hardness

Quartz at 7 is the most important stone on this side, because so many favourite crystals are quartz. Amethyst, citrine and clear rock crystal all share its hardness, and in many crystal traditions quartz is valued as an all-purpose stone believed to amplify intention, which is part of why it turns up in so many collections. You can explore the whole group in the Quartz & family section of the library. Harder still come garnet, emerald, topaz, ruby, sapphire and moissanite, all of which will score glass easily.

Rocks are trickier than single minerals. Granite, for example, is a mixture of quartz, feldspar, mica and sometimes other minerals, each with its own hardness, so a single Mohs number for it is only an approximation. Our article on granite’s Mohs rating explains why.

The Mohs Hardness Scale Glass Test, Step by Step

A glass scratch test takes a minute and costs almost nothing, but a little care makes the result trustworthy and keeps your fingers and your specimens intact.

1. Choose the right glass. Use a dedicated Mohs hardness scale glass plate, the kind included in many hardness kits, or a thick offcut with ground or taped edges. Never test on a window, mirror, glass shelf or tabletop you want to keep. Our guide to the Mohs hardness scale test kit covers what a good kit includes.
2. Protect yourself. Lay the plate flat on a folded towel so it cannot slide or crack against a hard surface. Wear thin gloves, and consider safety glasses if you are pressing hard. Keep children and pets away from the work area, and sweep up any glass flakes or mineral dust afterwards.
3. Clean both surfaces. Dust and grit can leave marks of their own. Wipe the plate and the stone with a dry cloth first.
4. Pick a discreet point on the stone. Use a sharp edge or corner on an inconspicuous part of a rough specimen. Avoid testing polished faces, faceted gems or the display side of a cluster (a group of crystals grown together on a shared base).
5. Press and drag. Hold the plate steady with one hand and draw the stone firmly across the glass for about a centimetre, with steady pressure rather than a jab.
6. Wipe and inspect. Rub the line with your thumb or a cloth. A soft stone often leaves a powdery trail that wipes away. A true scratch stays put. Run a fingernail across it: a real groove catches the nail. A hand lens (a small magnifier) helps with faint lines.
7. Confirm with a second check. If the result is borderline, compare against reference minerals from a kit, such as apatite (5) and orthoclase (6), to narrow it down.

A worked example: amethyst or fluorite?

Imagine you have two rough purple stones, roughly the same shade, and you are not sure which is which. Purple fluorite and amethyst can look surprisingly alike, especially as small broken pieces. Run the Mohs hardness scale glass test on each.

• The first stone glides across the plate and leaves a faint pale streak. When you wipe it, the streak disappears and the glass is smooth. That stone is softer than glass, consistent with fluorite at 4.
• The second stone bites into the glass with a slight grating feel and leaves a fine white line that does not wipe away and catches your fingernail. That stone is harder than glass, consistent with amethyst, a quartz at 7.

Fluorite also tends to break along flat, smooth planes, while quartz breaks with curved, shell-like surfaces, so the glass test and the shape of the broken edges together give you a confident answer. Hardness alone is rarely the whole identification, but it narrows the field quickly.

Is Glass Harder Than Steel?

For ordinary steel, yes. Reference tables place plain steel at about 4 to 4.5 and iron at about 4, both below the Mohs hardness scale glass value of 5.5. In Mohs terms, an everyday steel nail should not leave a true scratch on window glass.

Steel is not one material, however. Its hardness depends heavily on the alloy and on how it has been heat treated, and some hardened tools and blades can score glass. That variability is why experienced collectors do not rely on a single knife value. Instead, they use glass and a steel blade side by side to bracket a stone: if the blade scratches the specimen but the specimen does not scratch the glass, the stone probably sits somewhere in the middle of the scale.

Softer metals sit well below the Mohs hardness scale glass rating. Copper, for instance, rates about 2.5 to 3, and most plastics land around 2 to 2.5. A copper object makes a useful low-end check for very soft stones like selenite and talc.

Why soft things still seem to scratch glass

You may have noticed scratches on glass that appear after contact with something softer, such as a metal ring or a kitchen pan. Two things are going on. First, softer materials can cause tiny, permanent dislocations in a harder surface. These are real damage but are not counted as scratches under the Mohs definition. Second, and more often, the culprit is grit. Household dust and sand contain quartz particles at 7, and when a cloth or object drags those particles across a glass surface, the quartz does the scratching. The softer object simply carries it along.

Phone Screens, Tempered Glass and the 9H Label

The Mohs scale is not only a collector’s tool. Electronics makers use it to compare the scratch resistance of display parts such as the cover glass on screens. This is where a lot of confusion begins, because screen protectors are often marketed as 9H.

The 9H label does not mean Mohs 9. It comes from pencil hardness, a separate test in which pencils of graded hardness are drawn across a coating. A protector rated 9H is not in the same league as sapphire or corundum at Mohs 9. Tempering, meanwhile, mainly makes glass stronger against bending and impact; it does not turn it into a gemstone-hard surface.

For crystal lovers, the takeaway is simple. Screen glass may differ a little from the everyday Mohs hardness scale glass figure of 5.5, but quartz at 7 still outranks most everyday glass, so a loose tumbled stone (one smoothed and polished by rolling in a barrel with grit) rattling in a pocket beside a phone can leave fine marks on the screen. If you like carrying a stone, keep it in its own soft pouch. If you are building a small pocket collection, Spirit Magicka, the journal’s own crystal shop, is a place to start browsing.

Common Mistakes and How Glass Hardness Guides Crystal Care

Mistakes people make with the glass test

• Mistaking powder for a scratch. A soft stone can leave a streak of its own dust on glass that looks like a scratch until you wipe it. Always wipe and feel before deciding.
• Testing the best face of a specimen. A scratch test can chip or mark the stone as well as the glass. Use a hidden edge, or skip the test on finished jewelry and faceted gems entirely.
• Using household glass. Windows, mirrors and glass shelves can crack under pressure or end up permanently marked. Use a dedicated plate.
• Treating the scale as evenly spaced. Because it is ordinal, the jump from 9 to 10 is far larger than the jump from 1 to 2.
• Confusing hardness with toughness. Hardness is resistance to scratching; toughness is resistance to chipping and breaking. Emerald is harder than glass yet can be fragile because of its inclusions, and even diamond can chip along certain directions.
• Testing mixed rocks as if they were one mineral. On a rock like granite, the result depends on which grain you happened to press.

Using hardness to store and carry stones

The Mohs hardness scale glass line is a useful rule for organising a collection. Stones harder than glass, such as quartz, garnet, topaz and corundum, can scratch stones softer than it, such as calcite, fluorite and selenite. Keep the soft group in separate compartments or wrapped in cloth, and avoid tossing mixed stones together in one bowl or bag. If you display crystals on glass shelves, remember that a quartz cluster can mark the shelf; a felt pad or small fabric mat underneath protects both. For jewelry, stones below about 7 are more prone to dulling over time from everyday dust, which is why many people save softer stones for pendants and earrings rather than rings. Our guide to gemstone hardness and care and the talc-to-diamond gems chart go into this in more detail.

Hardness also says nothing about a stone’s traditional meaning or how much you might treasure it. A soft selenite wand can hold as much personal significance in a meditation practice as a hard sapphire, and many people find that knowing a stone’s real nature deepens their appreciation of the beliefs and stories attached to it. That pairing of folklore and geology is the heart of how this journal approaches crystals, as our editorial note explains.

Once you have tried the glass test a few times, you will start to feel the difference between a stone that skates across the plate and one that bites. From there, the rest of the library is a good next step: browse individual stones in the crystal library, or explore the wider range of minerals in Earth treasures. And if you want to add a few reference pieces to practise on, Spirit Magicka is the journal’s own shop.